An on-board unit in recent years is required to provide a plurality of services. For example, the on-board unit is required to conform both of a communication with the road-side communication device and a communication with another vehicle, and to conform both of a traffic data service and a billing service. For example, the on-board unit which can carry out both of a communication with the road-side communication device and a communication with another vehicle is disclosed in Patent Literature 1 (JP 2008-99085A). The on-board unit disclosed in Patent Literature 1 has two antennas to carry out diversity, and the two antennas are connected with either a transmitting and receiving section for the communication with the road-side communication device or a transmitting and receiving section for the communication with the other vehicle, by a switch. Note that as well-known to a person skilled in the art, in the diversity, a communication quality is secured by using a plurality of communication paths and using a signal on another communication path, even if a signal on one of communication paths is degraded.
WAVE (wireless access in vehicular environment) is proposed as a communication standard to provide a plurality of services to (a vehicle loaded with) the on-board unit. WAVE is a standard determined by revising for a mobile terminal, IEEE 802.11 which is a wireless LAN standard, and uses IEEE 802.11p in layer 1 (a physical layer), and layer 2 (a data-link layer). One feature of WAVE is in that the autonomous decentralized control (DCF) is carried out in which a transmitter chance is equally given to each terminal. In detail, in WAVE, each communication frame is divided into a plurality of time slots, each of which is called a “channel”. Each communication frame contains a single control channel and at least one service channel. A broadcast is carried out on the control channel, and a communication to transmit data to a specific transmission destination is carried out on the service channel. The communication terminal as a transmission source specifies a transmission destination communication terminal and a service channel on which a transmission is carried out, by using the control channel. The transmission destination communication terminal receives data from the communication terminal of the transmission source on the specified service channel.
One problem when a plurality of services are provided is in that the required communication speed and the required reliability may be different from each other among the plurality of services. For example, in the traffic data service, a large-volume data communication is requested whereas the required reliability may be relatively low. On the other hand, in the billing service, it is permissible that a communicable data quantity is less, whereas a high reliability is requested.
When the requested reliability is different among the plurality of services, it is considered to carry out the communication by using different communication speed ranges. In this case, when switching the service, it is necessary to switch the communication speed range. However, when a switching procedure is improper, a situation that an incomplete service is provided would occur. Especially, when the above-mentioned WAVE communication standard is used, it is not presupposed that the different communication speed range is used for the different service. Therefore, an issue of the switching procedure is fatal.